Line an 8x8 baking pan with aluminum foil and spray it with nonstick spray. Set aside.

In a large, nonstick saucepan, melt the butter over medium heat. Add the marshmallows and stir until melted, 3-4 minutes. Remove from the heat and stir in the chocolate chips, peanut butter chips and pretzel pieces, stirring until the ingredients are evenly distributed.

Using a spatula, press the mixture evenly into the prepared pan. Sprinkle with the salt. Allow to cool for about 20 minutes at room temperature.

Transfer to a cutting board. Peel off the foil and cut into two-bite squares using a sharp knife.